In order to provide output readings that will be indicative of certain characteristics of a fluid, earlier inventors have used probes that are disposed in contact with the fluid and that may be oscillated torsionally or reciprocally with respect thereto in order to determine the drag or other force that is exerted on the probe by the fluid. In most such apparatus power is utilized to oscillate the probe, and measurements of increasing or decreasing power requirements are used to determine the specific characteristics of the fluid then being tested. A primary usage of such type instrumentation is to determine the viscosity of a fluid either by testing samples thereof or by making near continuous readings of fluid moving past a probe sensor in a flow conduit. If a base power requirements value is established when the probe is disposed in a free air environment or in a liquid of known standard viscosity, an output reading showing a changed power requirement for maintaining the desired oscillating pattern for the probe can be indicative of the viscosity of other fluids being tested. The output readings of such instrumentation can be used to show changes in the shear modulus, viscosity, polymerization, flocculation, or coagulation properties of the fluid. In industrial applications such test apparatus may be used to control the intermixture of separate fluids in order to maintain a desired reading or to control polymerization processes in order to obtain a proper product.